For several years it has been initially assumed, and later confirmed, that a simple chemical must bind to an "epidermal protein" before it can develop immunogenicity with the resulting contact dermatitis seen clinically. However, this "epidermal protein" has not yet been specifically identified. Our preliminary studies with the tetrachlorosalicylanilide (TCSA) system suggest that serum albumin entering the intracellular spaces of the epidermis may be this long-sought protein. TCSA is an ideal hapten to study since it is a fluorescent compound with a very high index of sensitivity. We have demonstrated the presence of albumin in the epidermis and that TCSA will bind to it in vivo when irradiated with long-wave ultraviolet. It is planned to extend these initial studies and to determine if the hapten-skin albumin complex will be recognized by specifically sensitized lymphocytes, if that recognition is specific, if desensitization or tolerance can be produced to TCSA-skin albumin, to determine the specificity of cell transfer, and if TCSA applied to skin in vivo will bind to lymphocytes in draining lymph nodes.